Patent GB-541911-A (GB)
Title: Improvements in and relating to electrical musical instruments
IPC: G10H1/20
Filed: 1940-05-14
Abstract: 541,911. Electric musical instruments. HAMMOND INSTRUMENT CO. May 14, 1940, Nos. 8654, 8655 and 8656. Convention dates, May 18, 1939, Sept. 5, 1939 and Nov. 30, 1939. [Class 88 (ii)] [Also in Group XL] A key actuated electrical musical instrument of the kind in which a series of frequencyinterlocked oscillation generators, comprising a master oscillator and one or more controlled oscillators, is tuned to produce a series of harmonically related audio-frequency oscillations, and in which a corresponding series of switch-actuating, playing keys, each associated with at least one of said generators, is operable to select frequencies from those generated and cause them to be applied to an output circuit including a loud speaker, is provided with an additional series of switch-actuating playing keys, operation of any one of which produces a change in the tuning of the master oscillator and a corresponding frequency shift in the controlled oscillator or oscillators whereby another series of harmonically related audiofrequencies is made available for selective application to the output circuit under control of said keys. The two series of keys may be on separate keyboards, one set selecting the octave and the other set selecting any of the twelve semi-tones within the octave, but in the preferred form the keys are arranged as in an organ or piano in a continuous series extending over one or more octaves. Such a keyboard may be arranged in front of the usual keyboard of a piano, but shorter in extent, to enable organ-like notes to be played to pick out a melody. Means for securing a vibrato effect, a tone-control, swell, and variations in the attack and fading of the notes are provided as well as means for selecting the register of the notes controlled by the keyboard, whereby an extended range of notes is made available and octave coupling effects produced. It is not intended that chords, other than octaves, should be played except on instruments provided with more than one manual. Figs. 3 and 3a show the preferred circuit arrangements for an instrument covering six octaves. A master oscillator 10 is coupled as shown to a series of relaxation oscillators 12-17 of which the oscillator 12 is of the same frequency as the, oscillator 10 while the remainder oscillate at successively lower octaves. The master oscillatcr 10 is tuned by an inductance 34 in parallel with a series of condensers CC to CB in series. The other series of condensers in parallel with these are small trimming condensers. When all of the condensers are in circuit the master oscillator 10 and oscillator 12 oscillate at the frequency of the highest note B of the instrument, but as each of the condensers CB-CC# is successively shorted, the oscillators 10, 12 oscillate at frequencies which are successively a semitone less. The core of the inductance 34 has a variable air gap to enable the instrument to be tuned to the pitch of other instruments in an orchestra. Three octaves of playing keys are provided, the twelve keys of each octave being connected respectively to three bus bars 85, 86, 87, and the bus bar of any octave is connected to an earth lead 84 whenever any key in that octave is depressed. Depression of any key, except the B keys, also shorts one or more of the condensers CB-CC# to one or other of three bus bars 81, 82, 83 corresponding to the three octaves, the latter switches closing before the other switches. The bus bars 85, 86, 87 are in circuit with three relays 91, 92, 93 supplied with current from a source 300V through a voltage divider R98, R99, the circuit being completed whenever the corresponding bus bar is connected to the earth lead 84 by depression of a key. The relay circuits include normally closed switches 91a, 92a, 93a controlled by the relays so that the energization of any relay by a key in the corresponding octave prevents the energization of any adjacent relay should a key in the adjacent octave be accidentally depressed before the first key is released. This prevents undesirable effects in timbre and volume. The bus bars 81, 82, 83 are connected to normally open switches 91b, 92b, 93b controlled by the relays, closure of the switch connecting the corresponding bus bar to earth and consequently changing the tuning of the oscillator 10 by one or more semitones depending upon the number of condensers shorted. To enable the oscil-  lators 13 to 17 to remain stable at the lowered frequency the grid bias is correspondingly lowered. This bias is derived from a lead 54 connected to a high voltage source 300V through the voltage divider R14, R12, R13, a series of resistances in parallel with the condensers CC# to CB and with the resistance R14 being successively shorted as the frequency of the oscillation is lowered. The cathodes of all the oscillators 12-17 are connected to a common 10-volt source 10V. Two coupled inductances L2, L3 in the leads connecting the condensers and resistances suppress sparking at the switch contacts. Three other normally open switches 91c, 92c, 93c, controlled by the relays so as to close later than the switches 91b, 92b, 93b, connect a corresponding lead 101, 102 or '103 to an output lead 100 leading to the grid of a first amplifying valve 120. Four keys 104 to 107, each controlling three switches selectively, connect the leads 101, 102, 103 to the output leads 48 from any three successive oscillators of the series 12-17 through the resistances shown. Consequently the depression of one of the keys 104-107 selects the register of the instrument and depression of more than one effects octave coupling. By suitably selecting the values of the resistances connected to the leads 48 the keys 104-107 may be used to vary the proportions of the octaves played instead of effecting simple octave coupling. The output from the valves 12-17 being of "saw-tooth" character, a fundamental note accompanied by a series of diminishing harmonics is fed to the line 100 when a key is depressed. The output from the valve 120 is fed to the control grids of two variable-mu pentodes 124, 126 working in push-pull. These valves are normally biased beyond their cutoff point by a positive voltage maintained on their cathodes through a lead 96 connected to the junction of the resistances R98, R99. Whenever one of the relays 91, 92, 93 is energized its winding is put in parallel with the resistance R99 and consequently the voltage on the cathodes falls so that the audiofrequencies fed to their grids are amplified and passed on to the next pair of valves and so to the loud speaker 146. The rate of change of the bias is governed by a condenser C14 which softens the attack, a second condenser C16 and a control switch 132 enabling the attack to be softened still further. The rate of fading of the note is governed by a condenser C10 across the resistance R99 and this condenser may be variable in value. Swell.-The volume of the output is controlled by varying the voltage on the grids of the valves 124, 126, a knee-actuated lever enabling resistances R20-R24 and finally a direct connection 130 to be successively put in parallel with a resistance R19 of a voltage divider R18, R19 connected to a 50-volt source 50V. A resistor R17 and condenser C15 ensure that the change in voltage is gradual. Tone control.-A resistance capacity inductance network shown in Fig. 3 is controlled by a series of switches 150-155 to enable high or low frequencies to be accentuated or resonances introduced at 3,000, 1,000 and 200 cycles per second. Vibrato mechanism.-A vibrating reed 58 which is set in vibration when the instrument is started is maintained continuously in vibration by a magnet 64 and interrupter 66. Two inductances 74, 76 in parallel with a portion of the tuning inductance 34 of the oscillator 10 may be placed in circuit with a contact 68 by actuating either or both of two switches 70, 72. The contact 68 is intermittently grounded by the reed thus producing a variation in the tuning of the oscillator 10 at the periodicity of the reed. The various voltages at the points 300V, 10V, 2V &c. shown on the drawings are derived from a power pack described in connection with Fig. 2 (not shown). In a modification six sets of master oscillators with their interlocked relaxation oscillators may be employed, each being capable of being tuned through one semitone so that the twelve notes of each octave are available. Keyboard.-Fig. 1 shows a three-octave keyboard arranged in front of a piano keyboard so as to be playable simultaneously therewith. The keys are shorter than the standard keys and the black keys are narrower. A series of tabs 183 enables the various tone control, attack and fading control, register control, and vibrato control switches to be actuated, and a knee lever 129 actuates the volume control. A channel shaped guide 184 holds indicia 185 for identifying the tabs 183.